1. Field of the Invention
The present invention relates to an inner-focusing telephoto lens system having a half angle-of-view of about 4xc2x0 to 8xc2x0.
2. Description of the Related Art
An inner-focusing lens system in which a portion of the inner-lens groups (i.e., except the most object-side lens group) is moved in order to carry out a focusing operation (hereinafter, a focusing lens group) is suitable for an autofocus lens system. This is because the weight of the focusing lens group is light, thereby the load on the drive source is small. In particular, an inner-focusing lens system is preferable for a telephoto lens system since an increase in size of the optical system thereof, in theory, cannot be avoided.
However, the overall length of an inner-focusing lens system tends to become longer, because a space for allowing the focusing lens group to move has to be provided within the optical system. As a result, miniaturization of the inner-focusing lens system has been difficult, in comparison with an entire-advancing optical system and a front-group advancing optical system.
It is an object of the present invention to provide a small inner-focusing telephoto lens system having a half angle-of-view about 4xc2x0 to 8xc2x0.
In order to achieve the above object, there is provided a telephoto lens system including a positive first lens group, a negative second lens group and a positive third lens group, in this order from the object. Upon focusing from infinity toward the shortest photographing distance, the second lens group moves toward the image, and the telephoto lens system satisfies the following conditions:
1.35 less than f/f1 less than 1.80xe2x80x83xe2x80x83(1) 
1.30 less than |f/f2| less than 1.85xe2x80x83xe2x80x83(2) 
0.48 less than f/f3 less than 0.75xe2x80x83xe2x80x83(3) 
and
0.10 less than |L2/f2| less than 0.22xe2x80x83xe2x80x83(4) 
wherein
f designates the focal length of the entire telephoto lens system;
f1 designates the focal length of the first lens group;
f2 designates the focal length of the second lens group;
f3 designates the focal length of the third lens group; and
L2 designates the distance between the second lens group and the third lens group when an object at an infinite photographing distance is in an in-focus state.
In regard to condition (4), it is more preferable to satisfy the following condition:
xe2x88x920.22 less than L2/f2 less than xe2x88x920.12xe2x80x83xe2x80x83(4xe2x80x2) 
More particularly, the first lens group includes a positive first lens element, a positive second lens element, and a negative third lens element, in this order from the object. The second lens group includes a negative fourth lens element and a positive fifth lens element, in this order from the object. The third lens group includes a positive sixth lens element and a negative seventh lens element, in this order from the object.
In the case that the lens system is constituted by the above seven lens elements, the following conditions are preferably satisfied:
30 less than "ugr"p1xe2x88x922xe2x88x92"ugr"n3xe2x88x924xe2x80x83xe2x80x83(5) 
xcex8n3xe2x88x924+0.0016"ugr"n3xe2x88x924 less than 0.638xe2x80x83xe2x80x83(6) 
wherein
"ugr"p1xe2x88x922 designates the average Abbe numbers of the first and second lens elements;
"ugr"n3xe2x88x924 designates the average Abbe numbers of the third and fourth lens elements; and
xcex8n3xe2x88x924 designates the average partial dispersion ratios of the third and fourth lens elements indicated by the following equation:
xcex8n=(ngxe2x88x92nF)/(nFxe2x88x92nC); 
nxcex: xcex=the refractive indices of the g-line, the F-line, and the C-line respectively.
A diaphragm is preferably provided on the image side of the second lens group. More specifically, the diaphragm can be provided either between the second lens group and third lens group, or within the third lens group.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-241711 (filed on Aug. 9, 2000) which is expressly incorporated herein by reference in its entirety.